This invention relates to improvements in motor control circuits for use in electric power assisted steering systems, and in particular to pulse width modulation (PWM) control of multiple phase brushless motors in electric power assisted steering systems.
Control systems for PWM controlled electric motors, especially DC motors, generally need to measure the current through the windings or phases of the motor and this can either be done by means of separate current sensors for each of the phases, or by means of a single current sensor that is placed in the circuit so as to measure the total instantaneous current flowing between a D.C. power supply and the bridge circuit and motor combination. In a single current sensor system, the multiple motor phase currents are derived by offsetting the PWM patterns of the switches which apply the required voltage to each phase, and sampling the current sensor at appropriate points.
The measured currents are typically converted into the stationary d-q frame and then combined with a current demand signal, also in the d-q frame, indicative of the current that is demanded from the motor, to produce an error signal. The demand current in an electric power assisted steering system is generated as a function of the torque demanded from the motor. The torque demand signal is a principally a measure of the amount of torque the motor should apply to the steering to help the driver to turn the wheel.
The error signal represents the difference between the current that is demanded and the actual measured current. The error signal is fed to a controller which produces a set of voltage demand signals, also typically in the d-q frame, representative of the voltage to be applied to the motor that will best drive the error signal towards zero. The d-q voltages are then converted into PWM signals for the motor phases depending on which PWM strategy is used. The controller therefore acts to vary the PWM phase voltages in order to try to constantly minimise the magnitude of the error signal thereby ensuring that the motor current is as close as possible to the demanded current.
In a practical system the current controller will comprise a PI or PID or other type of feedback controller. The role of the current control is to modify the voltages applied to the motor with an aim of keeping the error signal value as small as possible. The controller forms a closed loop. Around the current controller the torque demand signal is produced by the torque controller, which forms another closed loop with a measurement or estimate of the torque in the steering as one input.
Motor drive circuits using feedback control and PWM are well known in the art. For example, WO2006005927, discloses a typical system and the teaching of that document is incorporated herein by reference.